Electrochromism is a color change in a material caused by an electrochemical oxidation or reduction reaction. Some electrochromic materials can be repeatedly switched between a colored and a non-colored state, while others exhibit multiple colored states. Conjugated electroactive polymers are one class of electrochromic materials where available colors can span the entire visible region of the electromagnetic spectrum. Further, the polymers have electrochromic activity outside of the visible in the UV, the near infrared, longer infrared and microwave regions of the spectrum. Electrochromism in conjugated electroactive polymers arises from the electronic transitions in the neutral polymer and electronic transitions created during oxidation or reduction. The color of the neutral state of the polymer is determined by the energy difference between the highest occupied molecular orbitals (HOMOs) that constitute the valence band and the lowest unoccupied molecular orbitals (LUMOs) that make up the conduction band, where the energy difference for the transition between the HOMO and LUMO levels is the bandgap. For many conjugated electroactive polymers, the bandgap lies in the visible region, yielding a highly colored polymer in the neutral state. Upon oxidation (p-doping) lower energy electronic transitions are created by the removal of electrons from the HOMO. The electronic transitions on p-doping occur at longer wavelengths that often extend into the infrared and lower energies. Such polymers having a neutral colored state and highly transmissive (to visible light) oxidized state are cathodically coloring polymers. On the other hand, there is a class of electrochromic polymers that have bandgap energies that occur in the ultraviolet (UV) spectrum. These polymers are essentially colorless in the neutral state and become colored upon oxidation where the lower energy midgap states allow for electronic transition to occur in the visible region of the spectrum. These polymers are referred to as anodically coloring polymers.
Two common electrochromic display devices (electrochromic devices) that use conjugated electroactive polymers are referred to as absorptive/transmissive and absorptive/reflective display devices. Absorptive/transmissive display devices contain two electrodes, a working and a counter electrode, which are transmissive to the wavelengths of interest, typically visible light. To maintain charge balance during switching and effective control of the color contrast exhibited by the device, electroactive polymers are coated at each electrode. For a device that switches between a colored and a non-colored state, as needed for electrochromic windows, a cathodically coloring polymer is coated at one electrode, while an anodically coloring polymer is coated at the other with an electrolyte layer positioned between the electrodes. While switching the bias of the device, the polymers act in a complementary nature with both switching between their respective colored and non-colored states where the resulting colors are a summation of those exhibited by the individual polymers. Because anodically and cathodically coloring polymers are used, the optical contrast can be high, but the variety of available colors is rather limited as very few polymer pairs are known that, when summed together, produce colors that are visually pleasing or of much utility as needed for commercial display or window applications. Generally, less saturated and more pastel-like or “earth-tone” colors are available.
Absorptive/reflective devices also contain two electroactive polymers coated onto electrodes, a working electrode and a counter electrode. However, the electrode materials are generally arranged in a configuration where their relative placement allows only one active layer to be seen as an outward facing electrode. In one common device configuration, the active working electrode is a gold-coated porous membrane onto which an electrochromic polymer of interest is cast. The metal electrode is necessarily porous to allow counter ion diffusion provided by an electrolyte to occur evenly between the polymer films during switching. Behind the porous working electrode is the counter electrode onto which another electroactive polymer layer is coated. This second polymer layer does not lend any optical properties to the device, but acts as a charge balancing layer. When a cathodically coloring polymer is used as part of the working reflective electrode, the device is colored when biased with a negative voltage; the color being that of the neutral electrochromic polymer. When the voltage across the device is positive enough to oxidize the polymer, it becomes transmissive, exposing the reflective electrode underneath.
Although conjugated conducting polymers offer a large color palette, in addition to a transmissive state, the colors exhibited in an electrochromic device incorporating these polymers are generally limited to two states and no systematic control of the wide variety of color is presently possible. Given that the simultaneous switching of a cathodically and an anodically coloring polymer in an absorptive/transmissive electrochromic device occurs with an additive color combination, many of the presently exhibited colors are not particularly pleasing for a display device. Furthermore, present devices do not generally allow the switching between multiple colored states and a transmissive state. Such color control is even less available for a reflective device where the colors of only a single electroactive polymer are available. Hence, there remains a need for a transmissive or reflective device that can display a variety of colors, particularly where a color combination is available that can be designed for a desired application.